Layout: separate visual component from Nebular root, Nebular root implementation

- nb-layout should not be required
- it should be possible to assign nebular root to any selector (?)
- nb-layout should be just a style component
- theme.service refactoring

```
currently we have one big layout component which is both visual layout (styles) and some API to communicate with scroll, resize and other document/window/_SO_CALLED_WRAPPER events and methods. It probably makes more sense to divide those in order to have smaller controllable peaces. Plus we have two modes - windowMode which moves scrolls to the layout - so we also should provide an api to communicate with scroll and that window events.

- Layout - windowWidth and related functionality should not be a part of NbLayout.
- Layout - nb-layout-column should not be required
```

Look into nebular CDK